The present invention relates to mixer circuits, and in particular, to mixer circuits and methods with improved spectral purity.
Mixer circuits are used in a variety of applications including modulation and demodulation, for example. FIG. 1 is a prior art mixer circuit. Mixer circuit 100 is sometimes referred to as a double balanced mixer because it includes two mixer circuits with outputs coupled together to common resistors. Mixer circuit 100 includes two sets of differential transistors 101-102 and 104-105 that each receive a first differential input signal (“in1+” and “in1−”). The differential transistors 101-102 (“M1” and “M2”) receive a bias current from transistor 103 that has a drain connected to the sources of both transistors 101 and 102 and a source connected to a current source 109. Similarly, differential transistors 104-105 (“M4” and “M5”) receive a bias current from transistor 106 that has a drain connected to the sources of both transistors 104 and 105 and a source connected to current source 109. Bias transistors 103 and 106 may also perform a voltage-to-current function. A second input voltage signal in2+ may be received at the gate of transistor 103, and a complementary input voltage signal in2− may be received at the gate of transistor 106. These signals (i.e., in2+ and in2−) will be converted to currents and coupled to the differential transistors.
One problem with existing mixer circuits, such as mixer circuit 100, is that device imperfections may result in spectral contamination. Ideally, it is desirable to match M1, M2, M4 and M5. Likewise, it is desirable to match M3 and M6. However, manufacturing variations may cause components of the circuit to be mismatched. Such mismatches may cause the output of the circuit may include a variety of unwanted frequencies. For example, mismatch in devices M3 and M6 may cause odd harmonic distortion and mismatches in the differential devices M1 and M2 or M4 and M5 may cause even harmonic distortion.
FIGS. 2-3 illustrate odd harmonics that are generated from mismatch in the circuit of FIG. 1. Waveform 201 is an example input signal, in1+. When in1+ is high and in1− is low, the current in transistor 103 (“M3”), Io1, is flowing through M1 and resistor 107 and the current in transistor 106 (“M6”), Io2, is flowing through M5 and resistor 108. When in1+ goes low and in1− goes high, the currents “commutate” (i.e., reverse) and the current in M3, Io1, flows through M2 and resistor 108 and the current in M6, Io2, is flows through M4 and resistor 107. Ideally, the currents in M3 and M6 are precisely the same. If Io1 and Io2 are identical, then the differential output voltage will be zero at all times as in1+ and in1− reverse polarity. However, if devices M3 and M6 are mismatched, the currents Io1 and Io2 will be different. Waveform 202 illustrates the change in the differential output voltage resulting from mismatched currents Io1 and Io2. When in1+ is high and in1− is low, the differential output voltage is given by the following equation:Vo_diff=out1−out2,out1=Vdd−Io1R, out2=Vdd−Io2R, Vo_diff=(Io2−Io1)R, When in1+ goes low and in1− goes high, currents Io1 and Io2 commutate and the differential output voltage is given as follows:Vo_diff=out1−out2,out1=Vdd−Io2R, out2=Vdd−Io1R, Vo_diff =−(Io2−Io1)R, If Io1 and Io2 are the same, the differential output will be zero. But if Io1 and Io2 are mismatched, Vo_diff will transition with the input signals in1+ and in1− as shown in waveform 202. Waveforms 204-206 illustrate the undesired spectral components that will appear at the output of the circuit as result of mismatch in Io1 and Io2. For example, waveform 204 illustrates a spectral component in Vo_diff having a frequency equal to the transition frequency of the input signals in1+ and in1−. Waveform 205 illustrates a second order harmonic of the fundamental frequency, which will be zero because a full cycle of waveform 205 will occur during a single half-cycle of the input. Waveform 206 illustrates a third order harmonic. FIG. 3 shows the odd harmonic components resulting from current mismatch in mixer 100. As shown in FIG. 3, mismatches in devices M3 and M6 will produce different current Io1 and Io2 that result in odd harmonic components at the output of the mixer. Such spectral impurities may adversely affect the operation of the system in which the mixer is used.
FIG. 4 illustrates even harmonics that are generated using the circuit of FIG. 1. Mismatches in differential transistors 101 and 102 and/or 104 and 105 may further contribute to the spectral impurity at the output. Waveform 401 shows in1+ and in1− as sinusoids. If transistors 101 and 102 are matched, the currents generated by these transistors in response to inputs in1+ and in1− will be the same. However, if these devices are mismatched, then the currents will be different. If one transistor (e.g., M1) has a larger transconductance than the other transistor (e.g., M2) because of mismatch, then the difference can be modeled as an offset voltage (“Voff”) at the gate of M1. Waveform 402 shows in1+ offset by an offset voltage Voff. Waveform 402 is illustrative in understanding the effects of mismatch on the output of the circuit. Waveform 402 illustrates that the crossover points (i.e., switching points) of in1+ and in1− are no longer coincident. When the crossover point of one pair of transistors (e.g., M1 and M2) shifts, then there will be periods of time when two transistors are turned on at the same time (e.g., M1 and M4) and are conducting current to one of the outputs. Likewise, for a short amount of time two transistors will be turned off at the same time (e.g., M2 and M5) and provide no current to the other output. This phenomena is illustrated in waveform 403, which shows the current in M1 generated by in1+. Comparing waveforms 402 and 403 it can be seen that the amount of time M1 is conducting current to the output is increased by an amount of time δt as a result of mismatch. Consequently, M1 may still be conducting current through resistor 107 when M4 turns on and conducts current through resistor 107. Thus, rather than an ideal situation, where Io1 and Io2 are alternately coupled to resistor 107 as shown in diagram 404, there is a short period of time, δt, where both currents are coupled to the output at the same time as shown in diagram 405. Plot 406 illustrates the current through resistor 107 (“IR107”) and resistor 108 (“IR108”) as a function of time. As shown in 406, mismatches in the differential devices may cause both transistors M1 and M4 to conduct current into resistor 107 for a short period of time, δt, resulting in a series of current pulses that occur twice per period of in1+ and in1−. Similarly, such mismatches may cause both transistors M2 and M5 to be turned off (i.e., zero current) for a short period of time, δt, resulting in a series of negative current pulses that occur twice per period of in1+ and in1−. The effect of these current pulses is shown in plot 407. If the currents Io1 and Io2 are equal, then the current pulses will generate differential output voltage (“Vo_diff”) pulses equal to 2IoR having a period of To. Since the current pulses occur twice for every period of in1+ and in1− (i.e., To is one-half the period of the input signal), the fundamental frequency of the output pulses will be twice the frequency of the input, and the output will include additional harmonics at even multiples of the input signal frequency.
In some cases, harmonic impurities resulting from manufacturing variances may be very small and effectively negligible. However, variations in the manufacturing process may cause different devices to exhibit different levels of harmonics. When the variances are sufficiently large, harmonic impurity may impact system performance, and in some cases may even cause the system to be completely inoperable. Thus, some portion of the circuits produced by the manufacturing process may have to be discarded, thereby affecting the “yield” of the process. Reducing the harmonic impurity caused by manufacturing variations would improve the production yield.
It is generally desirable to reduce the amount of harmonic impurity at the output of a mixer. Moreover, it is generally desirable to improve the yield of a manufacturing process. Thus, there is a need for improved mixer circuits and methods for improving the spectral purity of mixer circuits.